Of non-impact printers which are largely increasing their shares in the market nowadays, ink-jet printers are the simplest in principle, and also suitable for color printing. Of the ink-jet printers, so-called drop-on-demand (DOD) type ink-jet printers, which eject ink droplets only at the time of forming dots, are the most popular.
As a so-called piezoelectric ink-jet head using piezoelectric actuators among ink-jet heads for the DOD type ink-jet printers, there are a Kaiser type one as disclosed in Japanese Patent Publication No. 53-12138, a laminated piezoelectric actuator type one as disclosed in Japanese Patent Laid-Open Publication No. 6-8427, and a share-mode type one as disclosed in Japanese Patent Laid-Open Publication No. 63-252750.
In the piezoelectric ink-jet heads, motions of supplying ink to ink chambers from an ink supply source leading to the ink chambers, and a motion of ejecting ink droplets through nozzle holes formed in the ink chambers are executed by deforming the piezoelectric actuators with a voltage applied thereon, thus changing an inner volume of each of the ink chambers.
Conventional piezoelectric ink-jet heads are driven in a manner described hereafter. The wall faces of ink chambers are partially deformed by applying a voltage varying in a pulse waveform to the piezoelectric actuators, thereby increasing an inner volume of each of the ink chambers. In this step of driving operation, ink is supplied to the ink chambers.
Subsequently, the wall faces of the ink chambers are deformed in a reverse direction by stopping to apply the voltage to the piezoelectric actuators or by applying a voltage varying in a waveform of reverse polarity against the aforesaid waveform to the piezoelectric actuators, thus reducing the inner volume of each of the ink chambers. In this step of driving operation, ink is ejected through nozzle holes. Such a driving method is generally called the "pull-in shot" method.
FIG. 15 shows a pulse waveform of a voltage applied to the piezoelectric actuators and a displacement waveform of the piezoelectric actuators in a conventional method of driving an ink-jet head. In the figure, a waveform (a) indicates the pulse waveform of a voltage applied to the piezoelectric actuators, and a waveform (b) the displacement waveform of the piezoelectric actuators.
As shown in FIG. 15, the piezoelectric actuators which are in an initial condition over an interval of time T0 are charged with electric charge and deformed over an interval of time T1 when a voltage in a pulse waveform is applied thereto. Deformation of the piezoelectric actuators is accompanied by deformation of the walls of the ink chambers, increasing the inner volumes of the ink chambers and supplying ink into the ink chambers. Hereupon, free oscillation of the piezoelectric actuators as well as the ink in the ink chambers continues at a natural oscillation frequency even after deformation stops.
Electric charge that has built up in the piezoelectric actuators is discharged over an interval of time T2, and reverts to its initial condition. Hereupon, the inner volumes of the ink chambers are rapidly reduced, pressurizing the ink chambers and ejecting ink droplets out of the nozzle holes leading to the ink chambers. The free oscillation of the piezoelectric actuators continues at the natural oscillation frequency thereof centered around the initial position even after the ink droplets are ejected.
In the aforesaid conventional method of driving the ink-jet head, rapid supply of ink into the ink chambers is ensured, but on the other hand, the ink droplets are formed before the free oscillation that occurs in the piezoelectric actuators as well as the ink in the ink chambers damps out in case that the piezoelectric actuators are driven at a high frequency in order to increase printing speed. As a result, problems of the ink droplets breaking up or vaporizing have been encountered.
There is a method of driving an ink-jet head overcoming such problems described above by gradually increasing a voltage applied to the piezoelectric actuators while electric current is kept at a constant level. FIG. 16 is a diagram showing such a conventional method of driving an ink-jet head as described in the foregoing. In the figure, a waveform (a) indicates a waveform of a voltage applied to the piezoelectric actuators, and a waveform (b) a displacement waveform of the piezoelectric actuators.
Specifically, the piezoelectric actuators which are in an initial condition over an interval of time T0 are gradually charged with electric charge and deformed when a voltage varying in a waveform as indicated by the waveform (a) in FIG. 16 is applied thereto. Such deformation of the piezoelectric actuators is accompanied by gradual deformation of the walls of the ink chambers, and an increase of an inner volume of each of the ink chambers, thereby supplying ink into the ink chambers.
When a voltage in the waveform as indicated by the waveform (a) in FIG. 16 is applied to the piezoelectric actuators over an interval of time T2, electric charge is discharged therefrom, returning the piezoelectric actuators to their initial condition. Hereupon, the inner volume of each of the ink chambers is reduced, and the ink chambers are pressurized, ejecting ink droplets out of the nozzle holes. The free oscillation of the piezoelectric actuators as well as the ink in the ink chambers that occurs in the step of supplying ink is small in amplitude, and damps out in a short time.
However, in the method of driving the ink-jet head by applying a voltage in the waveform as shown in FIG. 16, the piezoelectric actuators are driven slowly in order to keep amplitudes of the free oscillations of the ink in the ink chambers as well as the piezoelectric actuators to a minimum. Consequently, as the time required for completing the step of supplying ink, that is, the interval T1 becomes longer, ink can not be ejected at a high cycle speed, causing a problem of the printing speed becoming slower.
Normally, in driving an ink-jet printer, the size of each ink droplet ejected from the nozzle holes is adequately adjusted according to the contents of printing.
For example, in the driving method described in the foregoing (for example, refer to FIG. 15), the longer the interval T1, the greater the amount of ink ejected becomes. However, with such a method, a period in case of continuous driving is lengthened due to a prolonged time needed for applying a voltage, resulting in a slower printing speed. Accordingly, the size of each ink droplet used to be adjusted in the past by increasing or decreasing the amount of ink ejected by means of varying a voltage applied to the piezoelectric actuators.
However, in case that the diameter of an ink droplet is adjusted only by varying the value of a voltage applied to the piezoelectric actuators, a problem arises wherein ink droplets of large diameter as targeted could not be formed because a sufficient amount of ink was not made available owing to a longer time required in supplying ink into the ink chambers for forming large-sized ink droplets than a time required for forming small-sized ink droplets.
In addition, it was difficult to enable an ink-jet head to acquire such a characteristic as capability of attaining linear variation in the diameter of each ink droplet, ranging from small to large, only by means of varying a voltage applied to the piezoelectric actuators and, furthermore, there was difficulty with controlling the voltage.
Furthermore, as free oscillation is caused to occur to ink inside the ink chambers by an ejection motion of ink, the position of a meniscus, that is, an ejection surface of ink in respective nozzle holes becomes unstable, and in case that the piezoelectric actuators are driven in such a condition to carry out a succeeding step of ejecting ink, fluctuation in both the diameter of each ink droplet and an ejection speed thereof results. Also, there is a risk of the occurrence of such a phenomenon as a succeeding ink droplet ejected being broken up when residual free oscillation still remains in ink. For this reason, a succeeding step of ejecting ink can not be carried out until the residual oscillation subsides, causing a problem of a printing speed being reduced.
In the light of the foregoing, it is an object of the present invention to provide a method of driving an ink-jet head, while solving such problems as described above. More specifically, the results stated hereafter are achieved by use of the method of driving an ink-jet head, according to the invention.
Rapid supply motions of ink into the ink chambers are ensured.
Ink droplets of consistent quality can be ejected at a high cycle by damping the residual free oscillation of the piezoelectric actuators that remains after completion of a step of supplying ink.
Ink droplets ejected out of the nozzle holes are formed in a required size with ease.
Ink droplets can be ejected steadily at a constant speed regardless of the size thereof and high speed cycle ejection motions of ink can be coped with without trouble.